Molecular simulation of the pore size distribution effect on phase behavior of methane confined in nanopores

An understanding of the phase behavior of hydrocarbons is important in the petroleum reservoir simulation. However, fluid phase behavior in a shale reservoir is substantially different from conventional behavior. Since fluids are stored inside nanopores of shale rocks, there is a strong interaction between the pore boundary and fluid molecules. Due to this interaction, the fluid molecules are distributed heterogeneously inside the nanopores and the phase diagram is shifted under confinement. Advanced theoretical procedures such as molecular simulations are needed to properly model the heterogeneous molecular distribution inside the shale nanopores. Previous molecular simulation studies of nanoconfined hydrocarbon phase behavior have been limited to single pore size models. However, shale rocks usually have a wide pore size distribution (PSD) and single pore-size models are not accurate enough to represent a real shale system. In this work, to understand the PSD effect on the phase behavior, a recently proposed molecular simulation method, gauge-GCMC, is used to generate phase diagrams based on two types of cylindrical models (single pore and multiple pores, including one based on Eagle Ford shale rock). In single pore tests, the pore diameter is changed from 4 to 10 nm. Our results for multi-pore systems show that with an increasing pore size, the phase equilibrium properties approach the bulk values. Also, smaller pores cause a more significant shift in the phase diagram. Our results show that the small pores are filled before the large ones, which means that liquid will first be condensed in the small pores. In the Eagle Ford case, the pore model is designed by discretizing PSD data from experiments. The results show that it is possible to use a single pore model with a 10 nm diameter to represent the pore system of this shale sample.